While there have been improvements in detection, treatment and prevention techniques, dental caries remains a prevalent condition affecting people of all age groups. If not properly and promptly treated, caries could lead to permanent tooth damage and even to loss of teeth.
Traditional methods for caries detection include visual examination and tactile probing with a sharp dental explorer device, often assisted by radiographic (x-ray) imaging. Detection using these methods can be somewhat subjective, varying in accuracy due to many factors, including practitioner expertise, location of the infected site, extent of infection, viewing conditions, accuracy of x-ray equipment and processing, and other factors. There are also hazards associated with conventional detection techniques, including the risk of damaging weakened teeth and spreading infection with tactile methods as well as exposure to x-ray radiation. By the time a caries condition is evident under visual and tactile examination, the disease is generally in an advanced stage, requiring a filling and, if not timely treated, possibly leading to tooth loss.
In response to the need for improved caries detection methods, there has been considerable interest in improved imaging techniques that do not employ x-rays. One method employs fluorescence wherein teeth are illuminated with high intensity blue light. This technique, sometimes termed quantitative light-induced fluorescence (QLF), operates on the principle that sound, healthy tooth enamel yields a higher intensity of fluorescence under excitation from some wavelengths than does de-mineralized enamel that has been damaged by caries infection. The correlation between mineral loss and loss of fluorescence for blue light excitation is then used to identify and assess carious areas of the tooth. A different relationship has been found for red light excitation, a region of the spectrum for which bacteria and bacterial by-products in carious regions absorb and fluoresce more pronouncedly than do healthy areas.
Applicants note some references related to optical detection of caries.
U.S. Pat. No. 4,515,476 (Ingmar) describes the use of a laser for providing excitation energy that generates fluorescence at some other wavelength for locating carious areas.
U.S. Pat. No. 6,231,338 (de Josselin de Jong et al.) describes an imaging apparatus for identifying dental caries using fluorescence detection.
U.S. Patent Application Publication No. 2004/0240716 (de Josselin de Jong et al.) describes methods for improved image analysis for images obtained from fluorescing tissue.
U.S. Pat. No. 4,479,499 (Alfano) describes a method for using transillumination to detect caries based on the translucent properties of tooth structure.
Among products for dental imaging using fluorescence behavior is the QLF Clinical System from Inspektor Research Systems BV, Amsterdam, The Netherlands. The Diagnodent Laser Caries Detection Aid from KaVo Dental Corporation, Lake Zurich, Ill., USA, detects caries activity monitoring the intensity of fluorescence of bacterial by-products under illumination from red light.
U.S. Patent Application Publication No. 2004/0202356 (Stookey et al.) describes mathematical processing of spectral changes in fluorescence in order to detect caries in different stages with improved accuracy. Acknowledging the difficulty of early detection when using spectral fluorescence measurements, the '2356 Stookey et al. disclosure describes approaches for enhancing the spectral values obtained, effecting a transformation of the spectral data that is adapted to the spectral response of the camera that obtains the fluorescent image.
While the described methods and apparatus are intended for non-invasive, non-ionizing imaging methods for caries detection, there is still room for improvement. One recognized drawback with existing techniques that employ fluorescence imaging relates to image contrast. The image provided by fluorescence generation techniques such as QLF can be difficult to assess due to relatively poor contrast between healthy and infected areas. As noted in the '2356 Stookey et al. disclosure, spectral and intensity changes for incipient caries can be very slight, making it difficult to differentiate non-diseased tooth surface irregularities from incipient caries.
Overall, it is recognized that, with fluorescence techniques, the image contrast that is obtained corresponds to the severity of the condition. Accurate identification of caries using these techniques often requires that the condition be at a more advanced stage, beyond incipient or early caries, because the difference in fluorescence between carious and sound tooth structure is very small for caries at an early stage. In such cases, detection accuracy using fluorescence techniques may not show marked improvement over conventional methods. Because of this shortcoming, the use of fluorescence effects appears to have some practical limits that prevent accurate diagnosis of incipient caries. As a result, a caries condition may continue undetected until it is more serious, requiring a filling, for example.
Detection of caries at very early stages is of particular interest for preventive dentistry. As noted earlier, conventional techniques generally fail to detect caries at a stage at which the condition can be reversed. As a general rule of thumb, incipient caries is a lesion that has not penetrated substantially into the tooth enamel. Where such a caries lesion is identified before it threatens the dentin portion of the tooth, remineralization can often be accomplished, reversing the early damage and preventing the need for a filling. More advanced caries, however, grows increasingly more difficult to treat, most often requiring some type of filling or other type of intervention.
To take advantage of opportunities for non-invasive dental techniques to forestall caries, it is necessary that caries be detected at the onset. In many cases, as is acknowledged in the '2356 Stookey et al. disclosure, this level of detection has been found to be difficult to achieve using existing fluorescence imaging techniques, such as QLF. As a result, early caries can continue undetected, so that by the time positive detection is obtained, the opportunity for reversal using low-cost preventive measures can be lost.
In commonly-assigned U.S. Patent Application Publication No. 2008/0056551, a method and apparatus that employs both the reflectance and fluorescence images of the tooth is used to detect caries. It takes advantage of the observed back-scattering, or reflectance, for incipient caries and in combination with fluorescence effects, to provide an improved dental imaging technique to detect caries. The technique, referred to as Fluorescence Imaging with Reflectance Enhancement (FIRE), helps to increase the contrast of images over that of earlier approaches, and also makes it possible to detect incipient caries at stages when preventive measures are likely to take effect. Advantageously, FIRE detection can be accurate at an earlier stage of caries infection than has been exhibited using existing fluorescence approaches that measure fluorescence alone. The application describes a downshifting method to generate the FIRE image.
Commonly-assigned copending PCT/CN2009/000078, entitled METHOD FOR DETECTION OF CARIES describes a morphological method for generating a FIRE image with reduced sensitivity to illumination variation.
Quantification of caries based on a digital image of a tooth such as a fluorescence image provides numerical information on the severity of lesion regions and can help dentists make and carry out treatment plans. It can be a useful tool in the longitudinal monitoring of caries for dentists to observe the evolution of each lesion area over time. U.S. Patent Application Publication No. 2004/0240716 has disclosed some methods for quantification of caries; however, the disclosed methods generally require manual extraction of lesion regions from sound tooth areas of the image by the user, and they are based on fluorescence-only images. Manual extraction of lesion regions from the image presents two problems. Firstly, the extraction process is slow, requiring the user to make many mouse clicks or to draw lines on the images to indicate the boundary of a lesion region. Secondly, manual extraction requires considerable caries diagnostic experience on the part of the user and is generally subjective. In addition, fluorescence-only images display incipient caries at relatively low contrast, further adding difficulty to the manual lesion extraction process. Therefore, in the disclosed methods, only compromised caries quantification results are achieved at best.
Commonly-assigned copending U.S. patent application Ser. No. 12/487,729 entitled METHOD FOR QUANTIFYING CARIES describes an improved method for quantifying caries. This method, however, partly relies on a key step of extracting a lesion area in a tooth image, which further partially relies on a key sub-step of identifying a tooth region.
There exist known methods for identifying a tooth region based on X-ray dental images using projection and active contour techniques, discussed by Jain et al., “Matching of dental x-ray images for human identification”, Pattern Recognition Vol. 37, pp. 1519-1532, 2004, and Chen et al., “Tooth contour extraction for matching dental radiographs”, ICPR 2004. These prior methods, however, are not applicable to a fluorescence image, reflectance image, or FIRE image because of significant differences in image content and characteristics. Thus, it can be seen that there is a need for a method for identifying a tooth region in a tooth image, particularly in a FIRE image or fluorescence image of a tooth.